I have a number of D7 sites that "broke" recently when I applied 7.39 and a few other updates. By "broke" I mean the site's CSS styling completely disappeared. I can easily fix by disabling CSS aggregation, so I know that's the cause.
At first I thought it might be a Omega 3.x problem since all sites initially affected, but then a site based on Bootstrap had the same problem. (The CSS doesn't always break right away).
What steps can I take to troubleshoot to determine the root cause?
Edit - to add more detail based on Clive's questions below:
- Page source does contain
<link>'s to aggregated CSS files (5 total)
- Aggregated CSS files do exist on server
- Status codes are 304 (4) and 200 (1)
- Folder permissions seem OK